Easy testing of Class-D Amplifiers

Class-D (or switching) amplifiers are popular, not only because of their light weight, but mainly because they are highly efficient, with less power distributed as heat. Their audio parameters, however, can be difficult to measure because of high-frequency noise produced at the output stage. NTi Audio solves this by filtering out these unwanted noise components with the FX100 Audio Analyzer plug-in module filter, providing reliable test measurement of class-D amplifiers.

Class-D amplifiers work by rapidly switching the output voltage on and off, thus creating a pulse waveform. The width of the pulses are made proportional to the input signal's instantaneous level. The power delivered to the loudspeaker, averaged over time, is the same as if the input signal had been amplified in the conventional way.

Signal and Noise
This switching operation creates considerable high-frequency noise, which must be separated from the useful signal in the audio band. These unwanted switching frequencies can range from 200 kHz to several MHz and modulate with the audio signal. In normal operation, the inductance of the speaker coil and the transmission range of the speaker remove this intermodulation. For testing the amplifier, on the other hand, the audio signal must be isolated from this noise with the FX-DF module low-pass filter.

Design and Connectors
The FX-DF module is equipped with two filter channels and fits into an available module slot of the FX100 Audio Analyzer. The output signals of the amplifier under test are connected to the filter module with Speakon plugs. A load (substituting the impedance provided by the speakers in normal operation) is connected to the test system through banana plugs. The filtered signals are automatically routed internally to the two analysis channels, making the test setup very compact and easy.

FX-Control Software
The control software of the FX100 incorporates the FX-DF filter module and provides an intuitive cabling wizard. From the filtered signals all the relevant parameters of the amplifier can be measured and recorded.